The present invention relates to an acousto-optic device particularly suitable for deflecting light beams in devices used for information processing, laser display, holographic devices, and the like, wherein the diffraction of light is obtained by application of an acoustic wave.
Acousto-optic devices used hitherto for deflecting light beams for optical information processing operate on the principle of the diffraction of light waves by an acoustic wave induced in a suitable optical crystal medium by a piezoelectric transducer. By changing the frequency of the acoustic wave, the deflection angle of the light beam is changed; while, by changing the amplitude of the acoustic wave the intensity of the deflected beam is controlled. A change of the polarization of the light wave can also result from the diffraction by acoustic wave. Depending on whether the device is used in systems for deflecting the light beams--deflectors--or in systems used for information processing, the most important parameters of an acousto-optic device are the diffraction efficiency, i.e. the ratio between the intersities of the deflected and incident beam, and the product of the time constant .tau. of the device with the width .DELTA.f of the frequency band. This bandwidth is determined on the one hand by the electric and acoustic properties of the piezoelectric transducer and on the other hand by the interaction bandwidth of the acoustic and light wave. An extremely large bandwidth of acousto-optic interaction can be attained in an optically anisotropic medium by making use of the so-called abnormal diffraction, in which the polarization of the deflected beam changes. In an optically uniaxial media the transverse acoustic wave is introduced, as a rule in a direction parallel to or perpendicular to the optical axis, whereas the direction of the incident beam is chosen so as to have the deflected beam emerging perpendicular to the optical axis.
In using the crystal types know so far, the described arrangement leads to rather high acoustic frequencies in the giga hertz range and the diffraction efficiency is low.
In the crystal of paratellurite (tellurium dioxide TeO.sub.2) strong rotation of the polarization plane of light can be used for the construction of an acousto-optic device with abnormal diffraction and high diffraction efficiency. The operating frequency of such acousto-optic device is relatively low, in the range of tens of megahertz, depending on the wavelength of light employed. In this case, however, the incident light must have approximately circular polarization.
An acousto-optic deflector utilizing the abnormal diffraction in a rotated tellurium dioxide crystal has been also known. The acoustic wave in the tellurium dioxide crystal propagates in the direction inclined at angle of 6.degree. from the [110] axis in the (110) plane, with the [110] direction of vibrations. This arrangement retains the high efficiency of interaction and eliminates the decrease of the diffraction efficiency in the middle of the frequency band. A disadvantage of this deflector is that the group velocity direction of the acoustic wave is inclined from the wave normal at a large angle, nominally 51.3.degree.. Consequently, an extremely large crystal volume is required for the construction of such a deflector. Besides, the deflectors made from the tellurium dioxide crystals cannot be used in the infrared spectral range beyond 5 .mu.m. Another disadvantage is the relatively high price for the tellurium dioxide single crystals of the required dimensions and quality. In addition, still another disadvantage of the acousto-optic device made from tellurium dioxide consists in the fact that the acousto-optic quality factor M.sub.2, which determines the diffraction efficiency for the diffraction by longitudinal waves, is rather small--about 1/30 of the value for the diffraction by transverse waves.
There is described in Dobrzhanskii et al CSSR Author's Certificate No. 170,007, an acousto-optic device made from a single crystal of univalent mercury halide, as described, which has high value of the acousto-optic quality factor M.sub.2 for both longitudinal and transverse waves. In addition, it transmits radiation even in the infrared spectral range with wavelengths larger than 5 .mu.m. A disadvantage of this device, however, is that, due to the low propagation velocity of the acoustic wave, sufficient frequency bandwidth can be obtained only by using a piezoelectric transducer of very small dimensions, whereby the requirements on the acoustic power density generated by the transducer are very severe.
There is also known two acousto-optic devices made from single crystals of univalent mercury halide, as described by C. Barta et al in U.S. Pat. Applications Ser. No. 968,930 filed Dec. 13, 1978 and Ser. No. 006,498 filed Jan. 25, 1979. The first device operating in the anomalous diffraction regime can be used even for infrared radiation of wavelengths larger than 5 .mu.m and can utilize efficient diffraction on longitudinal as well as on transverse wave; nevertheless, it does not suppress parasitic diffraction into the second diffraction order. This results in a drop of diffraction efficiency in the centre of the frequency band. The second acousto-optic device, which avoids the drop of the diffraction efficiency in the centre of the frequency band does not make possible the employment of diffraction on longitudinal acoustic waves. Such a result would have enabled the increase in the speed of the device several times.
The above mentioned shortcomings are voided in the acousto-optic device formed in accordance with the invention.